This program of research is directed toward identifying the viral genes which are expressed in viral transformed cells, and towardunderstanding the mechanism of regulation of those genes. First, we are attempting to construct a physical and genetic map of the oncogenic virus, polyoma. The physical map will consist of an ordered arrangement of unique fragments of the viral DNA generated by digestion with the bacterial restriction enzyme, endonuclease Z. Each fragment will be defined with respect to its gene content. This knowledge will provide an important working tool with which to investigate the role of each polyoma gene in the processes of viral growth, and viral transformation. Second, by the use of in vitro transcription with E. coli RNA polymerase, we have demonstrated that isolated chromatin is a specifically regulated template for transcription of the integrated SV40 genes in SV40 transformed mouse cells. We are presently attempting to determine whether chromatin of chicken cells is likewise specifically regulated for the transcription of the genes of the endogenous avian leukosis virus. These studies may provide a cell free system for investigating the mechanism for the repression of endogenous RNA tumor viruses.